1. Field of the Invention
The invention relates to a variable valve system of an internal combustion engine, and a control method thereof.
2. Description of the Related Art
In an internal combustion engine such as an engine for an automobile, a valve characteristic of an engine valve, for example, an intake valve is changed based on an engine operating state with the aim of optimizing an output from the engine and fuel efficiency in the entire engine operating region. Japanese Patent Application Publication No. 2001-263015 A discloses a valve characteristic changing mechanism which changes a valve characteristic. This valve characteristic changing mechanism is driven by a hydraulic pressure of oil discharged from a mechanical oil pump that is driven by an operation of an engine. The valve characteristic of the engine valve is adjusted to a target characteristic, which is set so as to be variable based on the engine operating state, by driving the above-mentioned valve characteristic changing mechanism, whereby the valve characteristic is made suitable for the present engine operation.
As a temperature of the oil for operating the valve characteristic changing mechanism increases, a viscosity of the oil is reduced, resulting in an increase in an amount of oil leaking from the valve characteristic changing mechanism and an oil supply passage through which oil is supplied to the valve characteristic changing mechanism. As such an oil leakage increases, the hydraulic pressure of the oil supplied to the valve characteristic changing mechanism decreases by a larger amount, and a response of the valve characteristic changing mechanism is likely to be delayed. Accordingly, for example, if the target characteristic is changed based on the engine operating state while the oil temperature is high, an adjustment-speed characteristic that adjusts an actual valve characteristic to the target characteristic may deteriorate.